The left column
compares histograms of the log of predicted-to-observed PGV ratios for the
Northridge earthquake.† The predicted
ground motions were computed using rock Greenís functions and site
corrections derived empirically.† In
this case we used the QTM site corrections employed by the southern
California TriNet ShakeMap. The histograms compare the performance for
quaternary, tertiary and Mesozoic sites.†
The red bars show that stations located in the Los Angeles basin are
systematically under predicted by factors of 2-3.† Slide 14 illustrates the predicted-to-observed amplitude ratios
in map view.

The right column
compares the predicted-to-observed ratios, where the predicted ground motions
were computed with site specific (hard and soft rock) Greenís functions with
no additional site corrections applied.†
The ratios were then classified according to the Geomatrix
scheme.† The red bars again show the
Los Angeles basin sites and it is observed that by using site specific
Greenís functions there is a considerable improvement in the amount of under
prediction (compare the quaternary and class D histograms).